Abstract
Left- and right-handers in humans coexist at least since the Paleolithic, and this variation in hand preference has a heritable basis. Because there is extensive evidence of an association between left-handedness and several fitness costs, the persistence of the polymorphism requires an explanation. It is not known whether the frequency of left-handedness in Western societies is stable or not. If the polymorphism is at equilibrium and maintained by frequency dependence, it implies that the fitness of left-handers equals that of right-handers. On the contrary, if left- and righthanders have a different fitness, the polymorphism will evolve. Using two large cohorts of French adults (men and women), we investigated the relations between handedness and several estimators of the reproductive value: marital status, number of sexual partners (of the opposite sex), number of children, and number of grandchildren. Left-handers seem to have disadvantages for some lifehistory traits, such as marital status (for women) and number of children. For other traits, we observed sex-dependent interactions with socioeconomic status: for high-income categories, lefthanded women report less sex partners and left-handed men have more grandchildren. These kinds of interactions are to be expected under the hypothesis that the polymorphism of handedness is stable.